Arc welding systems are currently used with robotic welding systems. The use of such robotic welding systems is typically to improve welding uniformity and reproducibility, and to increase the productivity and reduce the cost of welding. It is common that the robotic welder is responsible for carrying out, handling and/or manipulating the welding gun during a welding process. Some robotic systems have been developed which are preprogrammed to perform a fix sequence of motion and process actions. In such a system, the robotic welder merely repeats sequence of motions and processes for each weld. For such a robotic welder to operate efficiency, both the welding equipment and the workpiece must be precisely positioned with reliable repetition since any deviation in the welding equipment and/or position of the workpiece or the path of the joint to be welded will be result in an inferior weld.
Several robotic arc welding system has been developed to detect the area to be weld and to adjust the welding parameters and position of the robotic welder so as to apply a weld in a proper location. Many of these robotic welding systems include a vision system utilizing optical detection to detect weld and joint position and dimensional parameters of the workpiece. Such vision-guided systems assume that wire cast out of the contact tip does not vary in production. Examples of such welding systems are disclosed in U.S. Pat. Nos. 6,274,840; 5,558,785; 5,405,075; 4,920,249; 4,877,940; 4,737,614; and 4,296,304 all of which are incorporated herein by reference. These robotic welding systems which are incorporated by reference disclose various positioning techniques that can be used to increase the accuracy of the weld bead position on a workpiece. One or more of these robotic welding positioning systems can be fully or partially utilized in the present invention.
Another more common seam-tracking technique is a through-the-arc technique. In this technique, the robot weaves the wire inside the joint and uses current to detect the center of the joint. Again, this technique is heavily dependent on a consistent wire cast, which may or may not actually exist.
Although many of these positioning systems for robotic welders have provided satisfactory orientation of the welding gun relative to the welding location, these positioning systems do not account for welding wire wobble during a welding process. The welding wire that is being fed through the welding gun is typically fed from a spool, drum or reel. As such, the welding wire has a particular shape or memory as its being fed through a welding gun resulting in the welding wire moving laterally in various positions as its being fed from the end of a welding gun. Such movement of the weld wire results in weld wiggle of the weld bead on a workpiece.
Wire aiming accuracy is very important in robotic welding such as GMAW welding, to ensure proper weld location in a weld joint to obtain desired weld bead quality. In the past, factors such as robotic accuracy, fixture accuracy, part dimensional tolerance and distortion during welding have been for the most part corrected by prior art positioning systems. However, none of these positioning systems have so addressed or successfully addressed wire wobble problems. The wire wobble problem can be significant during the welding process and can be as great as an order of magnitude of the welding wire diameter. In the past, wire straighteners have been used in attempt to alleviate the problem of wire wobble; however, use of wire straighteners have not proven successful in dealing with such problems due to delicate setup and lack of standardization on a factory floor. Wire mechanical properties (e.g., stiffness, cast, pitch, packaging, feedability and delivery (e.g., liner condition, cable curvature)) can all contribute to wire wobble problems.
Several prior techniques have been utilized to account for the wire wobble of the welding wire as it is fed from a welding gun. Once such technique is to measure wire displacement by making multiple long beads on plate welds and observing the weld wiggle as an indication of wire wobble. This method is not proven effective to test large quantities of wire with good repeatability, and furthermore can be very expensive and time consuming. Another technique is to use a video camera with automatic edge detection image processing to record and measure wire wobble as the wire is exiting the weld gun. Although this method of detecting weld wobble has had some success, the video camera has a low frame rate and is adversely affected by lighting thus reducing the effectivity of detection. Laser sensors have also been used to measure wire movement under the contact tip, however, such past systems have been inhibited by the formation of a weld bead on a metal plate and the cost for such arrangements. Other methods of detecting wire wobble have also met with low success such as the use of a plasma arc to deflect the wire by arc force, or the use of induction heating of the weld wire which can be cost prohibitive.
In view of the state of the art with respect to detecting and correcting for wire wobble, there is a need for a wire detection system that quickly and accurately determines the position of a welding wire after the welding wire has been fed through the contact tip of a welding gun.